As is well known, Roots-type blowers include lobed rotors meshingly disposed in transversely overlapping cylindrical chambers defined by a housing. Spaces between adjacent unmeshed lobes of each rotor transfer volumes of air from an inlet port opening to an outlet port opening without mechanical compression of the air in each space. If the full axial length of the cylindrical wall surfaces of the chambers were to actually intersect, the intersections would form two cusps intermediate the chambers and extending between end walls of the chambers. In actual practice, all or most of the cusps have been removed by inlet and outlet port openings which respectively direct inlet air flow radially inward into the spaces between unmeshed lobes of each rotor and outlet air flow radially outward from the spaces.
U.S. Patent No. 4,768,934, which is incorporated herein by reference, and many other patents, e.g., U.S. Pat. No. 3,121,529, disclose Roots-type blowers having inlet and outlet port openings extending through the cylindrical walls of the housing for respectively directing all of the inlet and outlet air flow radially inward and radially outward.
The Roots-type blowers disclosed in U.S. Pat. No. 2,654,530 and SAE Technical Paper 870355 have modified inlet port openings wherein the inlet port opening in each extends through an end wall of the chambers. Some of the air from these modified inlet port openings flows axially into the spaces between the rotor lobes. However, most of the inlet air flow is intended to flow radially inward into the spaces from a channel extending axially from the inlet opening.
The above Roots-type blowers have functioned well as superchargers for vehicle engines even though they have suffered from rather poor volumetric efficiency. As taught in the above mentioned SAE Paper, test data therein demonstrated that volumetric efficiency of a Roots-type blower improved, particularly at low rotor speeds, by increasing seal time of the rotor lobes, i.e., the number of rotational degrees the rotor lobes defining each transfer volume are sealed off from the inlet and outlet port openings. As the ports were widened to provide increased flow area needed at high speeds, seal time and efficiency decreased. The inlet port opening disclosed herein provides substantially improved high speed and overall volumetric efficiency without loss of low speed efficiency.